1. Field of the Invention
The invention pertains to chemical processing, chemical process design, chemical process modeling, and laboratory apparatus, and in more detail to modular laboratory apparatus and associated components and associated computer systems and numerical models used in the study and design of reactive separation systems, and in particular as applied to reactive distillation.
2. Background of the Invention
Reactive Distillation (RD) integrates reaction and distillation processes and RD has demonstrated itself to be a profound development in the technology of industrial chemical production, particularly when distillation separation is involved. By integrating reaction and distillation processes, RD saves equipment, material, and energy costs. In typical application, RD is applied to reversible liquid phase reactions and at once significantly consolidates apparatus, reduces investment and energy costs, improves production times, provides improved purity, reuses heat of reaction, eliminates needs for some solvents and associated solvent recovery, and facilitates recovery of valuable materials from waste streams [1,2]. Further, RD can be additionally attractive due to valuable synergies. Examples of such synergies include shifts in chemical equilibrium conditions (for example as a result as the removal of products) and surpassing of usual distillation limits and reuse of exothermic heat generation as a benefit to the integrated reactions.
Notable History of Reactive Distillation
The earliest examples of processes in which RD was utilized did not attract attention as a notably different class of operation. The 1860's commercial Solvay ammonia recovery process is typically attributed as the first identified use of RD. This at least implicit use of RD was later followed by what amounts to RD production of propylene oxide, ethylene dichloride, sodium methoxide, and various esters of carboxylic acids [1].
It was for MTBE (methyl tertiary-butyl ether) that the RD process gained separate noteworthy status as utilizing a multifunctional element or step involving both a reactor and separator. This was dramatically followed by the now famous Eastman Kodak methyl acetate process that condensed an entire medium-scale methyl acetate chemical plant (comprising a reactor, nine columns, and extensive associated equipment and conduits) in a single Reactive Distillation multifunctional unit that directly accepted reactants and directly delivered pure products [1,2]. FIG. 1 depicts a representation of the Eastman Chemical methyl acetate RD apparatus.
In MTBE, ETBE (ethyl tertiary-butyl ether), and TAME (tertiary amyl methyl ether) etherification, RD can transcend equilibrium limitations and provide greater than 99% conversion [1]. A two-stage RD process providing simultaneous hydration and etherification can be used to produce di-isopropyl ether (DIPE) from propylene and water. Further, MTBE synthesis can be integrated with MTBE decomposition in a closed system [1] to provide pure isobutene where conventional distillation fails from too-closely clustered C4 boiling points. FIG. 2 (adapted from [5]) depicts a representation of the Hickey and Adams TAME RD apparatus.
In the Eastman Kodak high and ultra-high purity RD processes, an otherwise unworkable equilibrium limitation is traversed with no excess feedstock and is combined with sufficiently high reflux. The Eastman Kodak RD system proved to require only 20% of the capital investment and 20% of the former operating energy [2].
RD has been successfully applied on a commercial scale to etherification, esterification, selective hydrogenation, hydrodesulfurization, isomerization, and oligomerization, while hydrolysis, alkylation, acetalization, hydration, and transesterification have been identified as RD candidates [1,2]. Also cited as applications for reactive distillation are aldol condensation, amination, dehydration, and hydrolysis, among others.
More broadly, RD is one member of a larger evolving family of reactive separation processes that integration reactions and separations into a unified operation delivering various advantages. Other types of reactive separation processes include reactive chromatography, reactive membrane separation, reactive crystallization, reactive absorption, reactive adsorption, reactive extraction, and reactive stripping.
FIG. 3 shows a somewhat expanded view adapted from Aida & Silverston [6] depicting a few types of separating reactor systems differentiated according to phases of reactants, catalysts, etc. In Aida & Silverston RD is treated as lying at the intersection of gas, solid, and liquid phase considerations due to assumed role of solid catalysis, but more broadly need not be confined in this way.
Typical Situations Applicable to Reactive Distillation
RD is a natural candidate for consideration where shared temperature and pressure conditions facilitate both reversible chemical reactions and distillation-oriented phase equilibrium. RD performs some transcendent abilities (for example, overcoming chemical equilibrium product concentration limitations) through the use of distillation for removal of products from reacting feedstocks. To best accomplish this, reaction products should be at the density extremes (lighter and/or heavier) with the reactants having densities away from the extremes. In an ideal two-product case, one product is the heaviest among the products and reactants while the other product is the lightest, and the product boiling points should be at diametric extremes [2].
RD as defined thus far is applicable to limited situations where both chemistry and vapor-liquid (phase) equilibrium are sufficiently compatible. There are many types of RD arrangements and success stories within this range of situation. Some representative detailed documented examples include those classified in the following groups:                2 reactants, 2 products;        1 reactant, 2 products;        2 reactants, 1 product;        1 reactant, 1 product.(It should be noted that some systems, particularly those involving two-stage or multi-stage RD, have intermediaries that could be viewed as additional products, i.e., more than the 1 or 2 cited in the above list). In these examples, a variety of distillation column structures have been utilized according to the types of reactions and distillation arrangements involved [2].        
There are more specific and yet other typical requirements for “traditional” RD [2]:                On each tray, reactions and vapor-liquid (phase) equilibrium share the same temperature. Temperatures and pressures in the column affect both phase equilibrium and chemical reaction kinetics (It is noted, however, that mismatches between chemical reaction temperature ranges and vapor-liquid equilibrium temperature ranges can be handled by side reactors);        Individual and relative volatilities of components must permit reactants to be confined within the column and products readily removed and refluxed at the top and bottom of the column;        Reactions must be liquid phase as there is little hold-up (and hence little opportunity) for vapor phase reactions;        
Heats of reaction (or their dispersion) must not invoke excessive changes in vapor and liquid transport rates through the reaction zone.
Traditional Monitoring and Control Techniques for Reactive Distillation
Traditional RD systems as established employ the following types of process controls [2]:                Control of incoming flow rates of limiting reactant feeds;        Control of outgoing flow rate of bottom products;        Control of outgoing flow rate of distillate products;        Control of reflux ratios;        Control of pressure (controlled by condenser duty);        
RD systems as established employ the following types of process measurements [2]:                Temperature(s) at or between carefully chosen “control trays;”        Column pressure;        Column fluid base level.        
The universal types of RD control systems are those utilizing temperature measurements made on carefully chosen “control trays” to control incoming reactant feed rates [2].
Design Considerations for Reactive Distillation Systems and Processes
In the design of RD systems, the typical types of structural design parameters are [2]:                Column pressure (which influences RD far more significantly than conventional distillation and is one of the most fundamental design parameters);        Number of reactive trays;        Number of fractionating (rectifying and stripping) separation trays;        Reactant feed locations (unless near border of reaction zone, the location of light feed is far more significant than location of heavy feed);        Tray holdup;        Catalyst choice;        Packing materials, arrangements, and structures.Mathematical and Computer Models for Reactive Distillation        
Design methods for RD systems are evolving, particularly through the use of commercial software tools and incorporate increasingly sophisticated mathematical tools (for example, homotopy methods). Popular design methods done in isolation or employing commercial design software include:                Equilibrium conditions for both reactions and inter-phase phenomena [1].        Transformational methods [1].        RD line diagrams (noting that these typically require careful treatment to ensure thermodynamic consistency [1]).        
These and other design methods rely on underlying models. There are typically two types of models in common use, although each can be considerably embellished:                Stage (aka “Equilibrium”) models:                    Vapor-liquid phase equilibrium within each theoretical stage,            Mass balance for each theoretical stage,            Can include tray efficiency models—for example:                            overall (Fenske) efficiency (calculated across entire column),                average tray (Murphree) efficiency (calculated across entire tray),                point efficiency (calculated at some specified point),                packing tray efficiency metrics such as HETP (Height Equivalent per Theoretical Plate) and NTSM (Number of Theoretical Stages per Meter).                                                Rate-based models:                    Mass transfer handled by equations,            Heat transfer can be included,            Vapor-liquid phase equilibrium at interfaces only.                        
In general these models and other design methods must employ a firm understanding of properties of the reacting fluids, pressure/temperature phase equilibriums, azeotropic properties, and at times very specialized details peculiar to a specific problem. Regarding this and model embellishments, it is noted that model complexity increases significantly if mass transfer and/or reaction rates are included [1].
RD design inherits a number of finer-scale issues, design processes, and models from conventional distillation[3-5,8,9,10]:                Pressure variation static and dynamic) among trays (due to fluid blockage of vapor paths, etc.);        Fluid oscillation (sloshing) on trays (full-wave, half-wave, etc.);        Weeping;        Flooding;        Varying and mixed flow regimes (between weeping and flooding) [3,4] such as spray, froth. emulsion, bubble, and foam:        Liquid viscosity;        Stagnant regions at tray edges;        Packing (minimize pressure drop with increasing residency time);        Catalysis surface chemistry and processes (ion-exchange resins, metals, etc.).Use of Heuristics in Design of RD Systems and their Control        
RD work to date has led to a number of design heuristics. A few example design heuristics for the quaternary A+B=C+D system include (adapted from [2] pp. 529-534):                The feed of lightest reactant should be positioned higher on column than the feed for heaviest reactant;        If the relative volatility between reactants is small, position reactant feeds close together;        The reactant feeds are best separated further as relative volatility between reactants increases;        If relative volatility between lightest reactant and lightest product is large, the feed locations are best positioned upwards into the upper reactive zone;        If relative volatility between heaviest reactant and heaviest product is large, the feed locations are best positioned downwards into the lower reactive zone;        If rate constants are small, feed trays are best positioned with greater separation in the column; conversely if rate constants are large, feed trays are best positioned with lesser separation in the column.        
Additionally, various control heuristics are being established in industry, for example the choice of which trays are employed for temperature monitoring used to control incoming reactant flow rates (so that, for example, increasing temperature produces increasing flow rates [2]).
Incorporation of Additional Processing Structures into Reactive Distillation
In addition to the traditional RD configurations depicted in FIG. 1 and FIG. 2, workers in the field have incorporated additional functional elements to attempts to overcome shortcomings in traditional RD configurations for certain processes and to expand the types of chemical production to which RD can be productively applied. Some of these architectural and process variations include:                Side reactors (with and without heating) can be used to address mismatches between chemical reaction temperature ranges and vapor-liquid equilibrium temperature ranges [2]. Such arrangements permit the column to be operated at temperatures and (lower) pressures suitable for distillation while some chemical reactions are permitted to occur outside the column at temperatures and (higher) pressures suitable for reactions. Although separate reactors are employed, the resulting arrangement can still be considered RD because (i) reactive operations occur simultaneously with the distillation rather than sequentially with it, and/or (ii) reactions occur in the column. FIG. 4a depicts an RD column with a plurality of side reactors without the feed effluent heat exchanger (FEHE) elements. The lack of FEHE elements requires larger sizes of side reactor vessels. FIG. 4b depicts an RD column with a plurality of exemplary side reactors further supported by heat-exchange FEHE elements so as to reduce the size of side reactor vessels.        Control though use of “variable feed locations” (aka “feed tray manipulation” and “coordinated control”). FIG. 5 depicts an RD column with variable feed positions that can be controlled by valves;        Addition of Mass Separation Agents (MSA), also referred to as “entrainers,” to modify the relative volatilities of the components of an azeotrope it is able to interact with.Design, Prototyping, and Scale-Up        
A major challenge in the design of any distillation system is that of scale-up to a production facility based on a sequence of designs, models, laboratory prototypes, and pilot plants. Although the issues are well-known to one skilled in the art, a quick review of considerations, implementation heuristics, concerns, etc. for distillation system scale-up can be found in [11]. As challenging as scale-up is for conventional distillation systems, scale-up of laboratory-scale RD designs to commercial scale remains essentially impractical. A number of suspected issues are described in the Research Efforts section following below.
Although other approaches are possible, FIG. 6 depicts a representation of an example RD design process as relevant to the invention. Starting from conception, a high-level design may be used to create a detailed design. The high-level design and detailed design can be further refined using computer modeling. Computer models and detailed design can be employed at each of the steps of evolution from laboratory prototype to pilot plant to industrial plant capable of commercial operation.
Although RD has a well-appreciated pay-off for medium-scale to large-scale industrial plants when successfully implemented, it is also possible for RD to be attractive to small-scale production, for example in the small-scale production of limited-demand specialty and fine chemicals. Among the reasons for this are the abilities of RD to achieve high-levels of purity and for making certain types of reactions obtainable. In other cases, the equipment, materials, and energy savings provided by successful RD processes can also serve as an attractive factor in the small-scale production of limited-demand chemical products. Accordingly, FIG. 6 also shows in dashed lines an alternative evolution path for development of small-scale production and commercialization.
Research Efforts in Reactive Distillation
The synergies that provide RD so many potential advantages also make more general RD design, operation, understanding, and applicability very complicated [1]. First there are the inherent fundamentals of RD requiring situations where both chemistry and vapor-liquid (phase) equilibrium are sufficiently compatible. These must be merged with traditional reactor design, catalysis design, column design, tray design, and other related design issues for basic hardware and operation. Closed-loop control of the many controllable elements, and opportunities for further degrees of closed-loop control through the introduction of additional controllable elements and measurements, further expand the possibilities and complexity. Additional augmenting structures, such as those of FIGS. 4a, 4b, and 5 described in the previous section expand the possibilities and complexity yet further. Meanwhile, better mathematical and computer-based modeling and in the incorporation of applicable advanced mathematics each dramatically open the field to further advancement.
All of the aspects cited above stem from corporate and academic research efforts over the years. In some cases, the promise of RD has led to the creation of multi-institution programs involving both academia and industry. Two prominent pan-European programs 1996-1999 Brite-Euram (participants BASF, BP, Hoechst, Neste Oy, Snamprogetti together with Aston, Bath, Clausthal, Dortnund, Helsinki, and Moscow Universities) and the 2000-2003 Intelligent Column Intervals for Reactive Distillation (“INTINT”) project (AEA, BASF, DSM, Montz, Sulzer, together with Delft, Lappeenrannan, Manchester, Stuttgart, and Politehnica Universities and consortia/organizations PETROM, RDCRI, PAC-ICE, ICSO) were undertaken, each with a collective budget of ˜$4 million USD.
These programs were well-organized with useful and interesting deliverables. However, the inherently complex and readily evolvable field of RD is still far from comprehensive understanding. Additionally, important if not essential issues such as multiple steady-states, scale-up, spatiotemporal behavior, and others remain barely understood, and the many known, proposed, and inherently likely new architectural variations and control system innovations provide vast potentials that remain largely unexplored.
Almost all of the aspects listed above remain in active research with many problems and areas of poor understanding persisting.
Another important research area is that of RD system scale-up from laboratory prototypes. The ultimate goal is to skip costly and expensive pilot plants, but such scale-up of RD designs from lab to commercial scale is still essentially impractical due to reasons thought to potentially include [1]:                Interactions among mass-transfer processes and reaction processes can be more pronounced at larger scales;        Lab-scale packing can differ significantly from plant-scale packing (for example differing by a factor of 2 in the number of theoretical stages;        Incomplete catalyst wetting;        Temperature gradients in reactive packing;        Distribution irregularities of reactants;        Lab-scale separation efficiencies can differ significantly from those at plant-scale;        Lab-scale (˜50 mm dia.) column closely described by equilibrium-stage models, not true for plant-scale columns of only six-times larger diameter (˜315 mm dia.).        
Another area where many problems and areas of poor understanding persist is in complex mixture structures. For example:                Relative volatilities can be temperature dependent. This can create situations where a particular RD system design cannot operate because the role and ordering of at least one product volatility is exchanged with that of at least one reactant volatility (i.e., one or more reactants would tend to leave the top or bottom of the column while one or more products would tend to be retained in the reactive zone;        Vapor-liquid equilibrium model of quaternary mixture usually characterized by binary system information only. As a representative example of concern, there are 6 binary mixtures in the quaternary-mixture of 1-hexol+acetic acid+hexyl acetate+water, two of which are reactive. Sensitivity analysis can be used to demonstrate various issues that standard methods (for example group contribution methods for interpolation of mixture data) cannot account for;        Formaldehyde (a key C1 process compound) is usual manifest as an aqueous solution also including methanol, a system that turns out to be far more complex than a ternary mixture (in fact comprising more than 20 compounds in notable concentrations).There are many other examples, typically involving complex chemical reactions and likely requiring some method other than titrations (for example spectroscopy has been suggested [1]) in order to determine actual concentrations.        
An additional area where many problems and areas of poor understanding persist is in a collection of unexpected emergent RD issues and phenomena including [1]:                Multiple steady states;        Reaction/distillation oscillatory behavior;        Spatiotemporal phenomena;        Reacting azeotropes;        Startup processes, dynamics and stabilization.        
Further, ongoing R&D in RD has also led to a number of architectural and process variations:                Multi-stage RD systems, which although not uncommon are not fully understood and are in need of deeper study;        Use of side reactors (as discussed above)        Control though use of “variable feed locations,” aka “feed tray manipulation” and “coordinated control,” (as discussed above);        Use of Mass Separation Agents (MSA), also referred to as “entrainers” (as discussed above).        
Yet further, there may be opportunities wherein RD can be successfully and attractively extended beyond the traditional limited situations where both chemistry and vapor-liquid (phase) equilibrium are sufficiently compatible.
Each of the areas listed above, as well as many others, lie open for extensive future research.
Need and Opportunities for New R&D Tools for Reactive Distillation
Many academic and industrial programs continue to research RD further. However, although there are exceptions, many of these efforts and resultant publications appear to become increasingly formulaic and/or adding value by bringing in known techniques from naturally related areas (such as numerical analysis) and knitting approaches and models together (for example including extensive detailed hydraulics analysis). Such contributions are indeed of critical importance in building the needed foundations and framework of understanding, but are not in themselves vehicles of groundbreaking innovation.
In comparing the evolution of RD with examples in other technology evolution trajectories, it can be recognized that there is an important need for new tools, new means of discovery, and a new sense of adventure in RD research and development. It is to these needs for new RD tools, discovery, and adventure in RD research and development that the present invention is directed.
Overview and Example Benefits of the Present Invention
Overall, the present invention addresses the need for new tools for R&D in Reactive Distillation with the following contributions:                New tools for RD academic research and process development:                    Modular stages (for example, in the form of modular glassware) that can comprise one or more attachable or built-in sensors, controllable actuators, computer interfaces, and adjustable internal structures,            Packing and catalyst instrumentation,            New types of numerical tools,            Combinations of numerical tools and real-time monitored operation:                            To study and model physical phenomena,                To improve what can be done at lab scale in lieu of creating a pilot plant;                                                New types of RD measurement structures and measurement elements;        New types of RD control structures and controllable elements;        New types of RD control systems.        
A partial overview of the invention includes the following attributes:                Modular stages (for example, in the form of modular glassware) for lab-scale implementations of reactive distillation columns (and associated architectures);        Modular stages (for example, in the form of modular glassware) fitted with special structure control options:                    On/off and/or metered valves (automated or manual);            Tray parameters (magnetic servo) (to realize variable hold-up, for example),            Local thermal exchange;                        Modular stages (for example, in the form of glassware) fitted with special instrumentation options:                    Temperature sensors in one or more locations (including 1-D & 2D temperature sensor arrays for spatiotemporal studies),            pH sensors,            Ion-specific sensors,            Spectroscopy sensors,            Fluid level monitoring;            Imaging sensors:                            Video cameras and associated illumination,                IR cameras for thermal imaging,                UV cameras and associated illumination for spectral absorption and emission imaging;                                    gas/liquid flow-rate measurements;                        Computer interfaces for control and monitoring;        Instrumentation support/analysis software:                    Real-time statistical analysis,            Real-time image analysis/recognition,            Calculation of implied variables from measured data;                        Control algorithm library;        Integration of real-time simulation w/ real-time monitoring.        
FIG. 7 depicts a representation of some of the targeted value of some of the functional aspects of the RD innovations provided by the invention. Pertaining to the areas of conception, high-level design, and detailed design, the invention provides a broad range of new methods, new design options, and new potential applications. Pertaining to the areas of computer models and computer interfacing with laboratory-scale prototypes, the invention provides advanced instrumentation, modular architectures, new types of RD controls, reconfigurability, expanded numerical models, new methods combining modeling, simulation, and emulation. Pertaining to the area of pilot plants and industrial production plants, the invention provides the capabilities of new generations of RD architectures, components, measurements, and control. Pertaining to the area of laboratory-scale prototypes and their scale-up to small-scale production (for example for the commercial production of limited-demand specialty and fine chemicals), the invention provides advanced instrumentation, flexible customization through modular structures, new types of RD controls, reconfigurability, and support for new types of RD architectures via its modular structures.
The invention is expected to result in at least the creation of several new tools for RD research and design. This includes at least modular distillation and RD stage elements and interior components providing embedded instrumentation and associated computer interfaces and user interfaces. Additionally, it is very likely the integrated platform, bringing together a number of unique innovations and so many aspects of RD processes, mathematical modeling, and software systems will result in additional discoveries or new methods of RD, as well as s possible extensions to new types of production not known to have an advantageous RD approach or solution.
The invention provides for exploring scale-up process-divergence emulation via physical lab-scale emulation and/or numerical compensation.
In that RD has demonstrated it can save huge factors in operating energy and capital cost, as well as eliminating the need for so many solvent materials and processes as well as fewer points of failure, the extended new tools and understanding likely from the proposed research is likely to have valuable environmental impacts (energy and toxic pollutants) and cost reductions. In that distillation is among the great environmental offenders in chemical production, the likely innovations and results are an excellent target for investment.
Further, the integrated information system, simulation and lab-scale instrumentation framework provided for by the invention can be used to explore other less popular types of reactive separation [6,7] that in turn can be used in quite different settings, for example in lab-on-a-chip technologies.
Additionally, the instrumentation and numerical mathematical model implementations provided for by the invention can be used to shed new light on spatiotemporal phenomena in RD systems, perhaps finding ways to control and/or exploit it in commercial processes. Such results can also contribute a new chapter to the understanding and commercially exploitive use of self-organizing systems.